eel: string: Add documentation

This commit adds comments describing the functionality,
the parameters and the return values of the functions that were not
documented.

eel/eel-string.c

@@ -109,6 +109,15 @@ eel_str_capitalize (const char *string)
     return capitalized;
 }
 
+/**
+* eel_str_middle_truncate:
+* @string: (not nullable): input string
+* truncate_length: length of the truncated string
+*
+* Returns: (transfer full): a newly-allocated copy of @string with its middle
+* truncated and replaced with ellipsis to fit into @truncate_length characters.
+* If length of @string is already small enough, returns a copy of @string.
+*/
 gchar *
 eel_str_middle_truncate (const gchar *string,
                          guint        truncate_length)
@@ -151,6 +160,15 @@ eel_str_middle_truncate (const gchar *string,
     return g_strconcat (left_substring, ellipsis, right_substring, NULL);
 }
 
+/**
+* eel_str_strip_substring_and_after:
+* @string: input string
+* @substring: (not nullable): substring to use in search
+*
+* Returns: (transfer full): a copy of @string with the first occurence of
+* @substring removed, along with any trailing characters.
+* If @string is %NULL, returns %NULL.
+*/
 char *
 eel_str_strip_substring_and_after (const char *string,
                                    const char *substring)
@@ -175,6 +193,15 @@ eel_str_strip_substring_and_after (const char *string,
                       substring_position - string);
 }
 
+/**
+* eel_str_replace_substring:
+* @string: input string
+* @substring: (not nullable): string to be replaced
+* @replacement: string used as replacement
+* 
+* Returns: (transfer full): a copy of @string with all occurences of @substring
+* replaced with @replacement.
+*/
 char *
 eel_str_replace_substring (const char *string,
                            const char *substring,
@@ -285,6 +312,15 @@ get_common_prefix_length (char *str_a,
     return matching_chars;
 }
 
+/**
+ * eel_str_get_common_prefix:
+ * @strs: a list of strings
+ * @min_required_len: the minimum number of characters required in prefix
+ *
+ * Returns: (transfer full): the common prefix for strings in @strs.
+ * If no such prefix exists or if the common prefix is smaller than
+ * @min_required_len, %NULL is returned.
+*/
 char *
 eel_str_get_common_prefix (GList *strs,
                            int    min_required_len)